The present invention relates to the aircraft flight control art and, more particularly, to an aircraft flare control system which utilizes a flare envelope limiter.
A critical portion of aircraft landing trajectory is commonly known as flare out or flare. Flare is that portion of the landing trajectory between the fixed angle glideslope and aircraft runway touchdown. Thus, it is desirable, particularly for commercial aircraft, that the flare profile depart smoothly from the fixed angle glideslope approach, thereby providing a smooth transition to runway taxiing.
Aircraft automatic landing flare performance is determined by flare control circuitry which is commonly built into the autopilot. In order to make acceptable autolands under adverse environmental conditions, the autoland system must precisely control the flare path. Regulatory agencies define longitudinal and lateral "footprints" within which the aircraft must touchdown under various conditions. In general, simple basic flare control laws can meet such requirements. However, under certain severe wind conditions which induce aircraft drift, the simple control laws fail to guide the planes down within the specified footprints.
One solution to the above problem is to increase the flare control law loop gain, so that the system controls the flare path more precisely. A drawback to this approach, however, is that a high flare path loop gain creates an unacceptable high frequency oscillation on the control column or wheel which is objectionable to the pilot.
Yet another approach to providing an improved flare control is to modify the flare biases (constants) or adjust the biases depending upon flight conditions at flare entry. This so called "open loop" approach is limited in its ability to control the flare path, especially when a severe wind disturbance occurs during the flare maneuver. Also, extremely complex control laws are required to control flare paths in severe environmental conditions.
Thus, there has been a long-felt need in the aircraft guidance control art for an improved aircraft flare control.